Supplementary inflatable restraints (air bags) in automobiles utilize an accelerometer or acceleration sensor located in the engine compartment where it is subject to extreme stresses resulting from thermal cycling or thermal shock. Where the accelerometer is a mechanical type it is mounted in an open container along with a printed circuit board and other components and surrounded by an encapsulation material to protect the sensor and other components from the environment. In addition to the thermally induced stresses the sensor assembly is subject to water, salt and other chemicals which could have a deleterious effect on the sensor performance. Thus integrity of the encapsulation is vital to prevent intrusion of harmful substances.
While the above considerations are generally true of any electrical assembly exposed to such a harsh environment, an additional requirement is made of the encapsulation material in the case of an acceleration sensor: the material must faithfully transmit the acceleration of the vehicle-mounted container to the sensor within the container. Expressed another way, the encapsulation material must have a transfer function of one with respect to acceleration.
Heretofore, encapsulation material for acceleration sensors comprised a MDI (methylene di-p-phenylene isocyanate) and castor oil based urethane which is implemented by pot on sand (POS) processing which comprises pouring mixed urethane over hot sand contained in the sensor container, or by vibrating hot sand into the urethane. The cured mixture provides environmental protection of the sensor element. Such material has a number of disadvantages. It is extremely difficult to process, and any processing error could result in a low reliability assembly. Even when correctly processed the assemblies do not pass rigorous environmental testing for water intrusion into the sensing element after salt spray or thermal cycling testing. Conformal coating of the printed circuit board is required to protect the board from the sand in the potting material, and since substantial vibration is required to evenly distribute the sand in the urethane, some fixturing (as by an adhesive) is used to hold the sensor element in place.